An energetically and thermodynamically consistent Boussinesq model

Abstract

The Boussinesq approximation is a cornerstone of geophysical fluid dynamics, yet its thermodynamic and energetic underpinnings have remained ambiguous. In standard formulations, the links with the fully compressible Navier--Stokes equations are obscured, internal energy is only implicit, and the representation of diffusion and irreversibility remains \textit{ad hoc}. Here we derive a new Boussinesq model in a fully traceable way from the two-component compressible Navier-Stokes equations, ensuring exact energy conservation and consistent thermodynamics. Assuming a linear equation of state, our model treats density as a proxy for specific volume, distinguishes in-situ and potential temperature explicitly, and incorporates diffusive fluxes that homogenise the correct thermodynamic potentials, ensuring consistent non-negative entropy production. The result clarifies the status of gravitational potential energy, resolves ambiguities surrounding salinity--entropy coupling, and retains the small terms carriers of key information about thermodynamics. Alongside this, we introduce an exact thermodynamically soundproof (TS) model whose weak diabatic divergence highlights the role of compressibility effects in stratified turbulence. Together, these models provide a transparent framework that reconciles classical approximations with compressible energetics, offering better-defined pathways for analysing stratified mixing, mixing efficiency, and the energy budgets of geophysical flows.